The present invention relates to a method and apparatus for etching surfaces, and more particularly to a method and apparatus for generating jets and beams of atomic fluorine for etching such surfaces.
Etching the surfaces of materials is a basic process in the semi-conductor, micro-electronics, and opto-electronics industries. Such processes are used to fabricate microscopically small electronic and optical components in integrated circuitry, optical components in solid-state lasers and light-emitting diodes, and mechanical parts for machinery, such as micromotors and microgears. Etching can also be used to shape materials, like diamonds, that are extremely hard or brittle, and are therefore extremely difficult to machine using mechanical means.
Current etching techniques fall into the broad categories of "wet" and "dry" etching. In wet-etching processes, materials to be etched are exposed to liquids or solutions of aggressive chemical agents, such as hydrofluoric acid, which remove, by chemical attack, areas of the material that are not protected by an overlain "mask" that is chemically resistant to the etchant. Subsequent removal of the mask then leaves the pattern of the mask permanently engraved into, or "etched" upon, the surface of the material.
Wet-etching techniques suffer from several disadvantages, which include: use of extremely corrosive and toxic solutions; safe disposal of same; difficulty in keeping etched materials free of particulates having sizes comparable to the etching patterns desired (on the order of microns or less and getting ever-smaller); and perhaps most fundamentally, preventing the "undercutting" of the material being etched. The last disadvantage arises from exposing the material, covered by the erstwhile mask, to the etching solution from the side as soon as the etching process has begun to take hold below the surface. Consequently, "dry" etching techniques have been developed which attack material primarily from the side with the mask, which therefore serves as a shield even after etching of the material has begun in earnest. These dry-etching techniques are called "anisotropic" since they do not etch the material at equal rates in all directions.
To achieve anisotropic etching, the etching agent must be aimed in a specific direction towards the surface of the material being etched. Consequently, such etching processes are conducted in the gas phase, and are therefore "dry". Anisotropy is most commonly achieved by steering charged particles (ion-beam etching) or by electrically charging the material being etched to attract ions of opposite charge. These techniques are known collectively as "reactive ion etching", and are sometimes assisted by further addition of uncharged, but chemically reactive gases in "chemically-assisted reactive ion etching". Ions of the etching agent are most easily created by electrical, microwave, or radio-frequency plasma discharges through low-pressure gases. However, such processes often damage delicate structures in the material being etched owing to electrostatic charging effects and sputtering by high energy ions. In addition, even though etching rates are often very low (about tens of nanometers per minute or less) because of the low gas densities, these gas densities are still high enough that the reactive species may experience gas--gas collisions in the vicinity of the material being etched, thereby reducing the anisotropy of the etching process.
There is therefore a need for developing an anisotropic dry-etching technique wherein: (a) the chemically reactive etchant species is electrically neutral, thus avoiding surface charging and sputtering effects; (b) the chemically reactive etchant species can be accelerated to "hyperthermal" velocities corresponding to temperatures of 10,000.degree. K. or more, thus enhancing etching rates without sputtering; (c) the etching process operates at low background gas densities, thus avoiding loss of anisotropy due to gas--gas collisions; and (d) the etching process operates at high flux upon the material being etched, thus enhancing etching rates.
Ideally suited to these requirements is the "atomic beam" or "molecular beam" technique, wherein a stream of chemically reactive (uncharged) atoms or molecules is directed at the surface being etched. Rapid pumping of background gas being introduced in the beam results in the pressure within the beam being much higher than in the surrounding vicinity, thus enhancing etching anisotropy and possibly eliminating undercutting altogether.
Many dry-etching agents are based on chlorine- or fluorine-containing gases, which release chlorine or fluorine atoms or radical species containing them, upon dissociation or excitation in plasma discharges. However, atomic beams of chlorine, and especially fluorine, are difficult to generate in high fluxes owing to their reactive nature. The high gas reactivity, desirable for etching, necessarily destroys many conventional materials required for tubing within which to heat the gas for atom production.
Atomic fluorine is one of the most potent dry-etching agents, especially for etching silicon, the principal material of micro-electronics industries. The spontaneous etching of silicon by atomic fluorine and molecular fluorine beams has been studied in great detail, and the fluorine-silicon system is the most frequently chosen for directed beam studies. Exposure to hyperthermal beams, and to atomic fluorine instead of molecular fluorine, greatly enhances etching rates.
There are currently two common methods used for generating atomic fluorine: the fluorine-containing gas (usually carbon tetrafluoride, sulfur hexafluoride, xenon difluoride, or elemental molecular fluorine) is passed either through a plasma discharge tube (Stinespring, C. D.; Freedman, A. and Kolb, C. E., "An Ultrahigh Vacuum Compatible Fluorine Atom Source For Gas-Surface Reaction Studies", J. Vac. Sci. Technol. A, Vol. 4, No. 4, Jul/Aug 1986, pp. 1946-1947), or through a heated metallic tube. In etching studies the microwave discharge is the more common of the two methods because it avoids the problem of trace amounts of metallic fluorides that may condense on the target surface. In both cases however, the chief limiting factor is the low atomic beam intensity, which in turn limits expected etching rates. For plasma sources, this is due to the low gas pressure (typically a few torr or less), which result in typical fluxes for atomic fluorine of about 10.sup.14 -10.sup.16 atoms/cm.sup.2 -sec (corresponding to pressures of only 10.sup.-7 -10.sup.-5 torr). Also, the usual material of choice for the gas containment, a ceramic tube fabricated from alumina (Al.sub.2 O.sub.3), must be replaced regularly due to corrosion by fluorine (other crystalline materials such as quartz are also rapidly etched by hot fluorine).
Only two metals have been found to withstand the corrosive action of fluorine at temperatures high enough to induce substantial thermal dissociation. Using nickel at temperatures no higher than 1000.degree. K. allows intense beams to be generated, but with very poor atomic dissociation yields (about 15% or less). Fluorine rapidly destroys the nickel at higher temperatures, and even lower yields (5% or less) are usually tolerated to improve beam stability.
U.S. Pat. No. 4,734,152 discloses using iridium tubes to generate low pressure gas "jets", but only at the low fluxes and directionality obtainable from open-ended tubes. This gas jet technique is unstable however, since the iridium tube used for heating and conveying the fluorine is destroyed at temperatures below 1750.degree. K., while the alumina tube used to pre-heat the gas would be etched rapidly for temperatures above 1200.degree. K. if molecular fluorine had been used. The iridium source allowed brief and erratic use of sulfur hexafluoride as the atomic fluorine precursor, permitting measurement of relatively fast etching rates (hundreds of nanometers per minute), though it is not clear whether the active etchant was atomic fluorine or some lower fluoride of sulfur.
Finally, laser-induced dissociation of xenon difluoride has been used to produce intense, high-energy pulses of atomic fluorine. However, the beam lasts only as long as the laser pulse (about 5 nanoseconds), and its high instantaneous intensity is reduced by a factor 200 million since it can only be fired once per second.
It would be advantageous to develop a technique that generates atomic fluorine cleanly by heating molecular fluorine, directly inducing thermal dissociation. An atomic fluorine beam having a high local pressure and/or hyperthermal kinetic energy would drastically speed etching rates, and would improve etching anisotropy, without damaging surface structures by charging or sputtering effects. Using molecular fluorine as the atomic precursor avoids the production of possible by-products or particulates like elemental sulfur from sulfur hexafluoride or elemental carbon from carbon tetrafluoride. Additionally, dissociation of molecular fluorine requires only one-half the energy required for dissociating sulfur hexafluoride, and only one-third the energy required for dissociating carbon tetrafluoride.